An active matrix type liquid crystal display apparatus includes a pair of substrates opposing each other, a liquid crystal layer sandwiched between the pair of the substrates, and a display unit including a plurality of pixels arranged in a matrix pattern. One of the pair of the substrates includes scanning lines arranged along arrays of a plurality of pixel rows and signal lines arranged along arrays of a plurality of pixel columns in the display unit . The oriented state of liquid crystal molecules included in the liquid crystal layer is controlled by an electric field applied to the liquid crystal layer (JP-A-2009-296252 (KOKAI)).
Among others, liquid crystal display apparatuses of IPS (In-Plane Switching) system and FFS (Fringe Field Switching) system include a plurality of pixel electrodes arranged on one of the pair of the substrates in the matrix pattern and common electrodes opposing the plurality of the pixel electrodes, and are configured to control the oriented state of the liquid crystal molecules included in the liquid crystal layer by a lateral electric field generated between the pixel electrodes and the common electrodes. These liquid crystal display apparatuses have superior characteristics such as a wide view angle or low power consumption, and are widely applied to displays for TVs and mobile phones.
In recent years, there is an increasing demand for a user interface provided on a display surface such as a touch panel for improving operability, and products provided with a contact detecting element on the display surface of the liquid crystal apparatus are spreading in the market. For example, according to JP-A-2009-296252 (KOKAI), the contact detecting element and the liquid crystal display apparatus may be integrated, so that the liquid crystal display apparatus provided with a contact detecting function is provided at low costs.
In the above-described liquid crystal display apparatus, the orientation of the liquid crystal molecules included in the liquid crystal layer is controlled by voltages to be supplied to the common electrodes and video signals to be written in the pixel electrodes in sequence. In the configuration disclosed in JP-A-2009-296252 (KOKAI), the common electrodes also serve as wiring for detecting a change of electrostatic capacity caused by a contact on the display surface, and a plurality of the common electrodes are disposed electrically independently in the display surface.
The liquid crystal display apparatus and the method of driving the same of the related art will be described in detail below with reference to FIG. 5 to FIG. 8, and the problems will be clarified. The liquid crystal display apparatus here is assumed to be normally black and of the FFS system.
(1) Structure of Liquid Crystal Display Apparatus of Related Art
The structure of the liquid crystal display apparatus of the related art will be described with reference to FIG. 5.
As illustrated in FIG. 5, the liquid crystal display apparatus of the related art includes a pair of an array substrate (which is not illustrated) and a counter substrate (which is not illustrated) opposing each other, a liquid crystal layer LQ sandwiched between the array substrate and the counter substrate, and a display unit (which is not illustrated) including pixels PX arranged in the matrix pattern.
The array substrate is formed of a transparent insulative substrate (which is not illustrated), and pixel electrodes PE arranged on the respective pixels PX, scanning lines GL (GL1, GL2, GL3, . . . ) extending along the rows of the pixel electrodes PE, a scanning line drive circuit GD, signal lines SL (SL1, SL2, SL3, . . . ) extending along the columns of pixel electrodes PE, pixel switches SWP arranged at positions in the vicinity of points of intersection of the scanning lines GL and the signal lines SL, and common electrodes COM arranged so as to oppose the plurality of the pixel electrodes PE via the pixel electrodes PE and the insulating layer (which is not illustrated) on the transparent insulative substrate. The common electrode COM includes column common electrodes arranged along the columns of the pixel electrodes PE (hereinafter referred to as “column common electrode”) COM_Column and row common electrodes COM_Row arranged along the rows of the pixel electrodes PE (hereinafter referred to as “row common electrode”).
The pixel switch SWP includes a TFT (Thin Film Transistor) as a switching element. A gate electrode of the TFT is electrically connected to or integrated with the corresponding scanning line GL. A source electrode of the TFT is electrically connected to or integrated with the corresponding signal line SL. A drain electrode of the TFT is electrically connected to or integrated with the corresponding pixel electrode PE.
When an ON voltage is applied to the gate electrode of the TFT, electricity is conducted between the source electrode and the drain electrode, and a video signal is supplied from the corresponding signal line SL to the pixel electrode PE. A liquid crystal capacity is formed by the video signal applied to the pixel electrode PE, and a common voltage applied to the common electrodes COM_(COM_Column, COM_Row).
The pixel electrodes PE includes, for example, slits at a predetermined interval, and a lateral electric field is generated between the pixel electrode PE and the common electrodes COM_(COM_Column, COM_Row) arranged via an insulating layer. The oriented state of the liquid crystal molecules included in the liquid crystal layer LQ is controlled by the lateral electric field. Each pixel PX further includes an auxiliary capacity CS configured to be coupled with the liquid crystal capacity. The liquid crystal capacity is accumulated in the liquid crystal layer by the electric field applied to the liquid crystal layer. The auxiliary capacity CS is a capacity generated between the pixel electrode PE and the common electrodes COM_(COM_Column, COM_Row).
The common electrodes COM_(COM_Column, COM_Row) are wiring of the plurality of the common electrodes which are electrically independent, and also serve as wiring for detecting a change in an electrostatic capacity caused by the contact on the display surface.
In a display period, a common voltage is commonly supplied to each of the plurality of common electrodes COM (COM_Column, COM_Row). In a period in which the contact on the display surface is to be detected, independent detecting signals are supplied to the common electrodes COM (COM1, COM2, COM3, . . . ) respectively.
In one frame, rewriting of the liquid crystal display is performed in the same manner as the normal liquid crystal display apparatus in sequence on the basis of row scanning, and the detection of the contact on the display surface is performed during a vertical blanking period, so that the display of the liquid crystal and the detection of the contact on the display surface are both achieved. The detection of the contact on the display surface is performed on the basis of the detection signals as described in JP-A-2009-296252 (KOKAI).
During the display period, a common voltage common to the column common electrode COM_Column and the row common electrodes COM_Row, is supplied thereto by a common electrode drive circuit COM_Dry. The common electrode drive circuit COM_Dry includes a buffer circuit 1 and an amplitude control circuit 2. A high potential and a low potential of the common voltage are determined by the amplitude control circuit 2, and the buffer circuit 1 amplifies a current and supplies the same to the common electrodes COM_(COM_Column, COM_Row) after an adequate amplitude A has been set.
A switching element DEMUX_SW includes a demultiplexer, is turned ON in sequence during one horizontal period, and supplies the video signals output from one output terminal of a signal line drive circuit (which is not illustrated) divided temporarily into three signal lines (SL1, SL2, and SL3) to the same. The video signals supplied to each of the signal lines are supplied to the pixel electrode PE via a pixel switch SWP.
(2) Method of Driving Liquid Crystal Display Apparatus of Related Art
A method of driving the liquid crystal display apparatus of the related art will be described. In order to simplify the description of the problems of the related art, the driving method will be described in FIG. 7 with reference to FIG. 6, which illustrates a configuration of FIG. 5 simplified with a smaller number of pixels.
As illustrated in FIG. 6, the simplified liquid crystal display apparatus includes six scanning lines GL1, GL2, . . . GL6, and includes a column common electrode COM_Column, a row common electrode COM_Row1, a row common electrode COM_Row2, and a row common electrode COM_Row3 having the arrayed pixel electrodes PE as a plurality of the common electrodes COM.
As illustrated in FIG. 7, the gate signal output from the scanning line drive circuit GD is supplied to the scanning lines GL1, GL2, . . . GL6 to be driven in sequence by one horizontal period.
An AC driving of the liquid crystal is achieved by the signal line drive circuit (which is not illustrated) switching the polarity of the potential of the video signal to be charged to the pixel electrodes PE to positive and negative alternately for the common voltage from one frame to another.
When the potential of the video signal in the signal lines varies, in particular, when the video signal is charged by temporarily dividing the one horizontal period by a demultiplexer DEMUX_SW, a charging and discharging current is generated in the pixel electrodes PE of the selected scanning line, and hence the potential variation of the common electrode COM_occurs due to the capacity coupling.
However, the common electrodes COM_(COM_Row1, COM_Row2, COM_Row3, COM_Column) are provided as electrically independent wiring in the display surface as wiring for detecting the change of the electric static capacity caused by the contact on the display surface as described above, and hence have different and specific time constants, and the potential is converged independently according to the respective time constants.
Since the common electrodes COM_(COM_Row1, COM_Row2, COM_Row3, COM_Column) divide the display area in a plane, the potential variations thereof behave in conjunction with the scanning line included in the corresponding areas.
For example, the row common electrode COM_Row1 includes the scanning lines GL1 and GL2 in an area 3 and an area 5. Therefore, as illustrated in FIG. 7, between a period 1 (this period is one horizontal period) and a period 2, the gate signals GL1 and GL2 are input in addition to the coupling between the signal line and the common electrode COM, and hence direct transmission of electric charge with respect to the pixel electrodes PE occurs. Therefore, the potential of the row common electrode COM_Row1 notably varies. However, in other periods 3 to 6, the potential variation of the row common electrode COM_Row1 is caused only by the coupling between the signal line and the common electrode COM, and hence the potential variation is calm.
In the same manner, since the row common electrode COM_Row2 includes the scanning liens GL3 and GL4 in an area 6 and an area 8 between the period 3 and the period 4, direct transmission of the electric charge with respect to the pixel electrodes PE occurs in addition to the coupling between the signal line and the common electrode COM. Therefore, the potential of the row common electrode COM_Row2 notably varies. However, in other periods 1, 2, 5, and 6, the potential variation thereof is caused only by the coupling between the signal line and the common electrode COM, and hence the potential variation is calm.
In the same manner, since the row common electrode COM__Row3 includes the scanning liens GL5 and GL6 in an area 9 and an area 11 between the period 5 and the period 6, direct transmission of the electric charge with respect to the pixel electrodes PE occurs in addition to the coupling between the signal lien and the common electrode COM. Therefore, the potential of the row common electrode COM_Row3 notably varies. However, in other periods 1 to 4, the potential variation of the row common electrode COM_Row3 is caused only by the coupling between the signal line and the common electrode, and hence the potential variation is calm.
In contrast, since the column common electrode COM_Column includes all the scanning liens GL1, GL2, GL3, GL4, GL5, and GL6 in the area 4, the area 7, and the area 10 in the period 1 to the period 6, direct transmission of the electric charge with respect to the pixel electrodes PE occurs in addition to the coupling between the signal line and the common electrode COM, so that the potential of the column common electrode COM_Column varies notably and continuously.
(3) Problem of Related Art
As is clear from the description given above, an effective value of the voltage of the column common electrode COM_Column is smaller than effective values of the row common electrode COM_Row1, COM_Row2, and COM_Row3. Since the liquid crystal responds to the effective value of the AC electric field, the luminance of the pixels belonging to the areas of the column common electrode COM_Column is low.
In other words, as illustrated in FIG. 8, the area 4, the area 7, and the area 10 which belong to the column common electrode COM_Column are darker than other areas, and the display unevenness along the wiring of the column common electrode COM_Column occurs. In FIG. 8, the difference in luminance is indicated by a diagonal hatch and a cross hatch marked in the respective areas.
Although charging of the common electrodes in the display periods have been focused in the description here, the difference in effective value occurs from one display area to another even in a case where independent signals for inspection are applied to the column common electrode COM_Column and the row common electrodes COM_Row respectively in the vertical blanking period.
Accordingly, in view of such problems described above, it is an object of the invention to provide a liquid crystal display apparatus which prevents occurrence of display unevenness is avoided and achieves a desirable quality, and a method of driving the same.